Electromigration is the transport of material caused by the gradual movement of ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect of electromigration is an important consideration to take into account in applications where high direct current densities are used, such as in microelectronics and related structures. In fact, electromigration is known to decrease the reliability of integrated circuits (ICs) and hence lead to a malfunction of the circuit. In the worst case, for example, electromigration leads to the eventual loss of one or more connections and intermittent failure of the entire circuit.
The current density in interconnect structures increases due to scaling of the structures. This increased current density degrades EM (electromigration) related reliability. As such, as the structure size in ICs decreases, the practical significance of the EM effect increases. Thus, with increasing miniaturization the probability of failure due to electromigration increases in VLSI and ULSI circuits because both the power density and the current density increase. Also, it is know that in advanced semiconductor manufacturing processes, copper is used as the interconnect material which is subject to EM. Basically, copper is preferred for its superior conductivity.
In conventional structures, a metal cap is formed over the copper in an attempt to minimize copper migration, i.e., increase EM lifetime. The metal cap is formed by a selective electroless metal cap deposition process using, for example, CoWP. Although the selective electroless metal cap deposition process is selective to copper, unwanted deposition and/or rogue nucleation of the CoWP still results on the surface of the interlevel dielectric of the structure. That is, although the process is selective to copper, there is still a low deposition rate of the CoWP on the surface of the interlevel dielectric. This, in turn, results in high leakage and poor time dependent dielectric breakdown. In fact, the unwanted deposition and/or rogue nucleation can cause device failure due to shorting between adjacent copper lines.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.